Watch Series 4 Test Build

A test build of Realm Cocoa v3.9.0 built for watchOS with Xcode 10, including a slice for the new arm64_32 architecture.

To use with CocoaPods, install 3.9.0, build, then replace the copy of librealm-watchos.a found in the Pods directory with this copy and rebuild. For other installation methods simply replace Realm.framework and RealmSwift.framework with this build.